Chipboard or Plywood over floorboards

Not open for further replies.
23 Feb 2006
Reaction score
United Kingdom
I'm about to put floor tiles (ceramic or quarry), down in my bathroom.

Just wanted to know if Tongue and Groove (Floor graded moisture resistant) Chipboard was better than Plywood, for covering the floorboards, so as to avoid movement and grout cracking? I'll probably go for 22mm thick on either option.

Read on another thread that it's a good idea to leave a little gap between plywood sheets to allow for expansion. Is this necessary?

Sponsored Links
I should preface this response by saying that I've set more than my share of ceramic wall tiles, but have yet to set a single ceramic floor tile. However, I'm familiar with the issues that need to be addressed in both.

I'd contact the places that retail ceramic tile in your area and ask if they use a cement board or wood for the tile backer.

Here in North America, the advice generally given is to never tile directly over wood of any sort, even exterior grade plywood. The reason why is that wood swells and shrinks with changes in it's moisture content, and the grout lines of ceramic tile simply don't have the elasticity to accomodate any such movement.

What we do here instead is use a dimensionally stable material (like cement board or a gypsum based tile backer panel like Dens-Shield) over the wood. That way, if the wood gets wet, any movement in the wood will be accomodated by the twisting and bending of the fastners holding the tile backer down to the wood, as well as any crushing of the tile backer by those fasteners. But, the tile backer itself is dimensionally stable, and any movement of the wood below it will not translate into tensile or compressive stresses carried by the tiling or grout lines above it.

However, putting down cement based tile backer panels won't increase the rigidity of the floor as much as putting down plywood will, and rigidity is also important in ensuring the grout lines don't crack due to any bending of the floor when you walk on it.

Also, should the retail tiling outlets tell you it's fine to use wood as an underlayment over which to tile over, I'd definitely use plywood instead of chipboard. Plywood is much more rigid and would reduce any bending of the floor more than chipboard would.

Also, chipboard behaves much like a good breakfast cereal when it gets wet... it swells up and loses all it's strength. That's not what a good tile backer should do.


Regarding the gaps to be left between sheets of T&G plywood. Yes, leave the gaps.

You see, wood cells are shaped like long narrow cylinders that are closed off at their ends. As wood dries out, the cells walls become both thinner and stiffer, and this results in the wood shrinking considerably across it's grain, but by very little along it's grain. Lumber that's stamped "Kiln Dried" only means that the moisture content in the wood has been reduced to 19 percent or less. In a house, the moisture content will be less than 16 perecent. As a result, the lumber you buy will generally shrink as the water evaporates from the wood.

However, plywood consists of thin wood sheets that have been glued together, and the glue bonds better when the wood is dry. Consequently the plys of plywood are dried to a moisture content of only 4 percent before being glued together. And so, once you install that plywood and it absorbs moisture from the air in your house, it is much more likely to swell and expand than shrink and contract.

That's why they recommend leaving a gap between panels when installing plywood, but you also hear that the wood joists and wall studs in a new house will shrink as they dry out.
Ok, thanks for the advice. I guess I'll try and find a tile backer like the one you suggested.

Possibly the way to go is put plywood down first and then (if I can find it), put the tile backer over the plywood?

Thanks again for the advice. Very useful, and comprehensive!

Ply wood will be suitable, if you are overboarding the existing floor then 12mm will do but if you can afford the extra floor height then 18mm will be better. 12mm will only come with square edges, 18mm is available with T&G edges. Boards should be screwed at 200mm cts.

Alternativly you can screw something like Aquapanel to the floor, this will not be affected by any water that may get through the grout. Wickes stock it as well as the joint tape & screws.

You will also need flexible adhesive & grout, avoide the ready mixed "tile wooden floors" type and go for powdered materials

Sponsored Links
Just for my own curiosity...

If the original poster goes to the ceramic tile wholesaler or retailer, and...

a) they tell him/her to use T&G plywood as an underlayment for the tiling, but

b) the T&G plywood manufacturer recommends leaving a 1/8 inch gap between the plywood panels to allow for expansion after installation, then...

... is the original poster supposed to trowel the thin set mortar onto the plywood WITHOUT getting any of it in those 1/8 inch gaps?

Do professional tile setters trowel the thin set onto the plywood without getting any into those gaps?
there shouldn't really be any need to leave a gap between the boards ply is pretty stable and not suseptable to expansion along or across its faces its just delamination that is really the problem.
If you are worried about moisture getting through than use a paint on tanking system and then tile useing fastflex or similar, grout with flexi grout and bobs your mothers brother
Thanks everyone for your advice. I am a 'him' by the way Nestor :D

So just to sum up.

I think I'll go for 18mm thick T&G plywood with perhaps a layer of Aquapanel, then use a powerderd flexible tile adhesive on the tiles.

Is that about right, or have i gone to far using Plywood and Aquapanel?

I see Aquapanel thermal is 10 mm thick, so I'm guessing the normal grade is thinner.

Thanks Chappers, I meant to say I'd try your Fastflex adhesive.
Actually, I'm thinking that the reason you can probably fill the gaps if you do tile over plywood is simply because wood is a relatively soft material and doesn't expand with the irresistable force that other things do, such as the tremendous force with which water expands as it freezes or the force with which steel expands or contracts when fitting a heat shrink bearing on a shaft. Wood is basically air bubbles made out of cellulose.

Also, there are other things restraining the wood's LATERAL expansion if it gets wet;

1. the plywood is screwed down to the floor, for one thing

2. The grain direction alternates with each ply throughout the thickness of the plywood, so each ply would restrain the swelling of the plys on either side of it.

But, there really isn't anything restraining the plywood from becoming THICKER if it gets wet, and localized swelling of the plywood due to water penetrating into a cracked grout joint will cause more cracking, just as surely as lateral expansion will. (And so a cracked grout joint in an area that sees water can result in the ceramic tiling equivalent of a snowball rolling downhill.)

So, if it were me, regardless whether you tile over a cement board, or over plywood, I'd at least coat the plywood with a waterproofing membrane (like Laticrete) first to prevent any water from getting into the wood if it manages to get past the grout and into the floor.

But, like I say, I've tiled lotsa walls, but have yet to tile a floor. I'm just going according to the issues involved, not personal experience.
You really only need ply OR Aquapanel not both.

Aquapanel thermal is approx 6mm thick and not structural, regular Aquapanel is 12mm thick and structural so use the later. Thermal is a foam core with fibre mesh/cement surface, regular as cement/aggregate throughout with a fibre glass mesh on each side.

I don't leave any gaps when I over board with ply.

OK, thanks.

I'll perhaps get down to Wickes and have a look at this Aquapanel stuff first hand.

They're Victorian floorbloards, so would rather go to far in terms of stability, than not enough, and then have the hassle of trying to sort the tiles out after, if the grout stars to crack etc.

Maybe when I see the Aquapanel, I'll have more confidence in it, and just use that. As it is now, I don't have any idea of how strong/stiff it is.

Thanks again everyone, for your advice.

Cheers. Better get on with it now! :D
Hi DIYohwhy?

I have the same issue as you with age of house and bathrooom tiling.

So, how did you get on and what did you decide to use? Did it work?

I am thinking at present: 12mm plywood, covered by 6mm thermal aquapanel, then flexible adhesive to bed in tiles, flexible grout to finish.

1st of all ...
take up floor boards an check joists for damp , if damp use a wood preservative , then if the joists are in a state I would put in noggins at 2ft spacing (I would strenthen floor anyway )
use marine 18 mm ply or wbp ,
cut in handy sections of 3ft by 2ft by your timber merchant ,
screw down too joists an noggins ,every 10"
leave 3mm gaps for expansion (never happens but safer to do )
measure floor into a square an then quarter , then dry lay ,(get wifes approval ,a must !!)
mark the floor then do a quarter at a time ,an keep using a spirit level
take your time an get a BOSS Finish ,

use a powdered adhesive fastflex, an a similar grout same name brand (Bal is best ,I find )never use tub mixes
btw upstairs wooden floors dont transfer the cold like downstair concrete substrates can be ,
hope this helps .. :)

Well Moz sounds like he knows what he talking about. So follow his advice.

I haven't done mine yet, (seperate issue with towl warmer and pipes under the floor) but I have bought the materials.

I got 18mm thick ply. Has to be specifically for floors, I think as Moz has suggested, mine was wbp.

I also bought 12mm thick Aquapanel, the normal stuff NOT the thermal. As I recall the thermal is thinner (Strangely).

It might be a bit of overkill using Ply and Aquapanel, but the way I see it, I only want to do this job once. So the ply will even up the floor, and the Aquapanel will protect the ply. However, the Aquapanel is very strong and rigid, so you'll have to judge it yourself. Better safe than sorry. Mines a converted Victorian house, so any additional thickness on the floor would benefit my neighbour below :)

Hope that helps, don't forget to buy the special Aquapanel screws and joint tape.

Good luck! Tell me if you learn anything useful when you do it.

P.s. Moz is also right about the powdered adhesive fastflex, from what I've been told.

as long as DIYohwhy?

thinks Im ok trust my advice ...;)

your on overkill with the aquapanel ..but go ahead lol...

btw you can use a piece shaved at one end as a shim from the landing floor up to the tiled bathroom ,as that height difference may be a big step :)
:D ha ha lol

Well fortunately the bathroom is on the lowest level and hence has stairs going down into it, so the height/ thickness of the floor isn't an issue.


DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Not open for further replies.
Sponsored Links